Segmentation fault - c

Question

#include <stdio.h>
int main(void)
{
/* an array with 5 rows and 2 columns*/
char* a[5][2];
int y, p;
for(y = 0; y < 5; y++)
{
for(p = 0; p < 2; p++)
{
scanf("%s", a[y][p]);
}
}
int i, j;
/* output each array element's value */
for ( i = 0; i < 5; i++ )
{
for ( j = 0; j < 2; j++ )
{
printf("a[%d][%d] = %s\n", i,j, a[i][j] );
}
}
return 0;
}
I've been getting a Segmentation fault as an output of this program after inserting 2 strings. Can anyone tell me what's wrong with my code?

Answer 1

Problem:
You're declaring 10 uninitialized pointers here:
char* a[5][2];
And then trying to fill them with data:
scanf("%s", a[y][p]);
This is wrong. You need to allocate memory before copying data.
Solution:
I would do this in a more sexy way, but the quick solution would be:
#define MAX_LEN 100
char a[5][2][MAX_LEN];
/* ... */
scanf("%s", a[y][p][0]);

Answer 2

Just a hint on helping troubleshoot. Hope this will help the future readers who are new to pointers.
A better way to troubleshoot a segmentation fault is to run it with a debugger like gdb.
e.g
Compile your program with gdb (You need gdb installed on your host)
gcc -ggdb Test.c -o Test
Then run it with gdb
gdb ./Test
In your case, you will see an output like this. It'll go to the gdb prompt.
Then type run or r, it will run the program. Then it asks for an input. Type your input value. Then the segmentation fault occurs. Now you can see your backtrace by typing backtrace or bt. You can see which line causes your crash. You can see the code by list. You can go to any line by typeing list <line>. Go through a GDB Guide to find out more commands.
It tries to access a pointer, probably invalid that's why it crashes. Then find out why it's invalid. Probably you didn't initialize it or didn't allocate memory. So the easiest fix would be to declare it as an array (rather than a pointer array) like #Andrewjs mentioned in his answer.
Reading symbols from /tmp/Examples/Test...done.
(gdb) run
Starting program: /tmp/Examples/Test
for 1
for 2
10
Program received signal SIGSEGV, Segmentation fault.
0x005c5c3d in _IO_vfscanf_internal (s=0x6d1420, format=0x804867e "%s", argptr=0xbfffe984 "\020\204\004\b", errp=0x0) at vfscanf.c:840
840 *str++ = c;
(gdb) backtrace
#0 0x005c5c3d in _IO_vfscanf_internal (s=0x6d1420, format=0x804867e "%s", argptr=0xbfffe984 "\020\204\004\b", errp=0x0) at vfscanf.c:840
#1 0x005cebbb in __scanf (format=0x804867e "%s") at scanf.c:35
#2 0x0804850d in main () at Test.c:16 <-- Your program's last call
(gdb) list
835 }
836 #else
837 /* This is easy. */
838 if (!(flags & SUPPRESS))
839 {
840 *str++ = c; <-- Crash point
841 if ((flags & MALLOC)
842 && (char *) str == *strptr + strsize)
843 {
844 /* Enlarge the buffer. */
(gdb)
For advance debugging of applications This may help